Add fuzzer pass-through config to tester.toml

[MattWindsor91]

This issue tracks exposing the configurable bits of act.conf (and any other fuzzer we may support in the future) through the tester.toml file. This would need:

• an idea of fuzzer configurability (probably letting us disable fuzzing entirely);
• some additional code in the act.conf maker;
• some thought perhaps as to how to make this generic (but not too much thought, YAGNI etc).

This might look as follows (assuming a fix to #28):

[fuzz]
  style = "act"
  run.cmd = "act-fuzz"
  run.args = [ "-v" ] # ideally there'd be a 'run.verbose' flag or something, but this is an example
  [fuzz.config]
    params."action.cap.lower" = 20
    params."action.cap.upper" = 200
    flags."action.pick-extra" = "100:1"

[MattWindsor91]

The fuzz.config part of this is done now, but the specification of how to run the fuzzer hasn't (mostly because it's tied into the way the fuzzer runs act binaries).